This sort of information is locked away by the USP so that we patients cannot access it!
However, searching found what claims to be a copy of the USP document and that says:
It may contain a suitable diluent such as Lactose, Sodium Chloride, Starch, Sucrose, or Dextrose.
My memory is claiming there might be one or two other substances are specifically permitted but I cannot find confirmation. I'll update if I can find out more. But as written, it seems to allow anything that is considered "suitable" without definition of that.
